It is true that fire protection in tunnels demands another approach than fire protection in buildings”, judges Paul Buggenhoudt, Promat’s Tunnelmanager in Belgium.

“Whereas fire protection in buildings mainly aims at a safe evacuation of the occupants of the building, fire protection in tunnels involves an additional dimension, namely the economic and public interest of maintaining the tunnel operational. Tunnels are built on the most interesting locations from an economic point of view: under waterways, in densely build-up areas, on important junctions, etc. The failure of a tunnel would not only involve considerable costs of repair, but also additional traffic jams on our already congested roads, causing a lot of wasted time for all road users involved.” Considering furthermore that a failing tunnel structure may end in the collapse of the buildings above it or floods in case of subways under waterways, one can easily arrive to the following conclusion with regard to fire protection in tunnels, according to Paul Buggenhoudt: “In case of fire, Passive Fire Protection in tunnels has to enable all users to exit the tunnel in safety, allow easy access to the emergency services and prevent the tunnel structure from being damaged irreparably.

Several serious fires in European road and rail tunnels during the past years have led to the publication of a paper by PIARC (World Road Association) named “Recommendations of the Group of Experts on safety of road tunnels”. This paper prescribes among other things that all possible structural, technical and organizational actions have to be taken to guarantee the safety in tunnels.

These measures, which relate to both tunnel infrastructure and vehicles, have to meet the latest technological standards and apply to all concerned, in particular road users, traffic control, emergency services, etc. Prevention has been put forward as the primary objective to be met: the prevention of critical events, which can endanger human life, the environment and/or the tunnel infrastructure.

The secondary objective aims at the reduction of the consequences of the incident by creating the ideal prerequisites for:

people involved in the incident to rescue themselves

the immediate intervention of road users to prevent greater consequences

ensuring efficient action by emergency services

protecting the environment and

limiting material damage.

Apart from this report, the European Union also was aware of the widely divergent security regulations and standards, rules and structural requirements in its member states. Since this situation was considered as ‘confusing’, the EU decided to issue its first Directive in this respect: “Directive 2004/54/EC of the European Parliament and of the Council on minimum safety requirements for tunnels in the Trans-European road network”. One of the merits of the Directive is that it draws the attention of the member states to the need of minimum safety requirements (prevention) and measures in order to reduce the consequences of an incident. The Directive prescribes that these measures have to be determined on the basis of a risk analysis performed by an independent party.

The aforementioned information shows clearly that safety measures in road tunnels have to refer to all aspects of the tunnel: its structure, technical equipment (such as fire detection and extinguishment), traffic situation, supervision, etc. As a result it is obvious that all necessary passive and active fire safety measures have been taken in an important tunnel project as the Liefkenshoek railway connection.

Paul Buggenhoudt: “Of course an effective detection system and appropriate fire-extinguishing equipment are essential, but only in addition to the necessary passive fire protection measures, which are at the basis of structural fire protection. In the opinion of Paul Buggenhoudt, active and passive measures are complementary. “They complement each other. Passive fire protection measures are operational from the moment on they are installed, active fire protection measures are put into operation when a fire starts.” In the Liefkenshoek railway connection all necessary active fire protection measures have been taken (see a detailed analysis in our previous edition). The construction itself is protected against the effects of a possible fire by means of Cafco FENDOLITE®-MII fire protective coating.

Considering that the strength of concrete is reduced to zero at temperatures of 800°C-900°C and air temperatures during a hydrocarbon fire in a covered environment can rise up to 1350°C (according to RWS – Rijkswaterstaat), it is obvious that concrete has to be protected against fire. Considering furthermore that air temperatures during a tunnel fire can rise up to more than 1100°C, following damaging mechanisms may be expected when the concrete has not been protected:

Spalling: the water contained in the concrete turnes into steam due to a sudden temperature ris and cannot escape in time. This causes explosive spalling of the concrete surface. The higher the compressive strength class of the concrete, the sooner the spalling may be expected to occur. The spalling criterion for standard concrete is assumed to be 380°C, whereas for high-strength concrete this criterion falls to 225°C or even lower. The spalling can also be progressive, given that after a piece of concrete has fallen of, another part is exposed to thermal chocks and high temperatures.

Reduction of the tensile strength of the steel reinforcement causing permanent deformation.

Occurrence of temperature gradients between the hot and cold side of the concrete, causing cracking at the invisible, inaccessible, hardly reparable cold side.

Cracks and detaching inside the concrete.

Rijkswaterstaat

Already during the seventies the Dutch Ministry of Infrastructure and the Environment (Rijkswaterstaat) has asked TNO (Dutch Organisation for Applied Scientific Research) to test the behaviour of fire in tunnels. At that time several small-scale tests were performed, which have laid the foundation of the RWS-fire curve (see also Edition 33 of Fireforum Magazine). Meanwhile more elaborate and targeted tests have proved that the RWS-values are very realistic. In collaboration with TNO, Rijkswaterstaat has established a test procedure for fire protection in tunnels.

Two different tests have been elaborated in this regard: the “spalling test” and “thermal insulation test”. According to the “spalling test” method, the temperature of the concrete may not exceed 380°C, whereas the temperature of the steel reinforcement has to remain under 250°C in order to avoid concrete spalling. These requirements of Rijkswaterstaat only refer to concrete, which has been poured on the spot. Since precast concrete is much less porous because it contains finer filling materials, the temperature of prefab concrete may not exceed 200°C. As a consequence it is harder for the water to escape from the concrete, which increases the risk of spalling (due to the generation of steam).

Liefkenshoek: fire resistant insulation

The concrete of both the two newly drilled tunnels (Liefkenshoek) and the 35 years old (but never used) Beverentunnel has been protected against fire and heat by means of a spray solution. The requirements to be met correspond to the RWS-requirements described previously. Furthermore the adhesive strength of the fire resistant spray had to amount to a minimum of 0,3 N/mm² after 28 days. In order to meet these requirements the protective layer has to be 27.5 mm thick. Each of the tunnel pipes is approximately 5.975 m in length and 7.30 in diameter. The fire resistant coating had to be applied at a great pace (5 days out of 7 and 24/24 hours. A daily progress of 675 m² had to be made in order to keep up with the planning (total 174.446 m²). The Beverentunnel is 1.200 m in length and has a total surface of 20.472 m² to be coated.

In this tunnel a daily progress of 450 m² per 12 hours was required. Also the mandatory control parameters were clearly described in the specifications (adhesion control every 150 m², thickness control every 100 m², water stream test every 700 m²).

Edition 6 of Fireforum Magazine mentions a tunnel project in Dubai, where calcium silicate boards (PROMATECT®-H) have been used to protect the tunnel against fire. Why apply a fire resistant spray in the Liefkenshoektunne then? Paul Buggenhoudt: “A board solution is no option in circular tunnels, such as the Liefkenshoektunnel. A sprayed mortar can be applied faster and more easily. The board solution could have been an option for the Beverentunnel, because it’s a rectangular tunnel. Strictly speaking boards are the best solution for newly built rectangular tunnels, because they can be used as lost formwork, which makes them doubly profitable. The major advantage of a coating is the fact that both its thickness and the temperature gradients can easily be controlled (avoid cracking).

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